Fiber optic devices are tested after manufacture to determine their optical characteristics. For example, attenuators may be tested to determine the actual attenuation of the device, and fiber optic couplers may be tested to determine the coupling ratio at selected wavelengths and polarizations.
FIG. 1 is a conventional arrangement for testing a fiber optic coupler. The fiber optic coupler 10 includes fiber optic leads 12a, 12b, 12c, and 12d that are connected to a testing apparatus to determine the optical characteristics of the fiber optic coupler 10. The fiber optic coupler 10 is tested by providing light from a light source, for example a laser 14, into one of the fiber optic leads 12a at a predetermined input power, and measuring the output from the fiber optic leads 12a, 12b, 12c, and 12d using detectors 16a, 16b, 16c, and 16d, respectively. The detected power levels are used to develop a transfer matrix, which identifies the transfer of power from points A to B, A to C, A to D, and the back reflected power from A back to A. The back reflected power (A to A) is detected by a detector 16a via a coupler 18, also referred to as a test coupler or "zero" coupler. The detected power levels (P.sub.i) are also used to determine the insertion loss for each optical path, namely the fraction of power entering an input port A that exits a corresponding output port (i.e., L.sub.AC =-10 log(P.sub.A /P.sub.C)), and the excess loss (L.sub.E) which specifies in decibels the amount of power lost between the inputs and outputs (L.sub.E =-10 log(P.sub.C +P.sub.D)-10 log(P.sub.A)). A specification for the sensitivity of the fiber optic coupler 10 with respect to the tested wavelength and the tested polarization state can also be determined for the transfer matrix by varying the wavelength and polarization state of the input light.
The testing apparatus of FIG. 1 has a number of associated problems. A primary problem is that substantially all of the testing must be done manually, where a technician fusion splices the fiber optic coupler 10 onto the testing apparatus at splicing points 20a, 20b, 20c, and 20d. After testing the input A, the technician needs to cut and resplice the leads to test input B, and possibly inputs C and D. Hence, poorly-spliced connections may cause undesirable variables in the measured readings.
In addition, use of an additional coupler 18 to test for back reflectance introduces additional uncertainty as to the amount of power actually entering into the fiber optic coupler 10 at point A, since losses may occur in the test coupler 18. For example, some of the laser energy input to the coupler 18 may be lost as energy directed to the detector 16a instead of the input point A of the coupler 10. The coupler 18 may also have its own back reflection characteristics between the laser 14, the detector 16a and the fusion splice 20a. Depending on the characteristics of the test coupler 18, the laser light may encounter a -3 dB loss before reaching the input point A, and back reflected light from point A may encounter another -3 dB loss during travel via the coupler 18 to the detector 16a. Hence, the coupler 18 may add at least -6 dB of loss due to laser light passing from the laser 14 to the fusion splice 20a via the coupler 18, and loss of the back reflected light from optical fiber 12a passing through the coupler 18 to the detector 16a. The losses associated with the coupler 18 might be determined by splicing in a detector in place of the splice 20a or using a wide area detector to measure the output of the coupler 18 before performing the splicing of coupler 10. However, such a procedure is a labor intensive and may produce additional variances during calibration.
Moreover, the testing arrangement of FIG. 1 is a labor intensive process, requiring manual splicing to add lasers, detectors and couplers to the device under test. The repeated fusion splicing and disconnections of the optic testing apparatus of FIG. 1 with different configurations for the device 10 under test adds additional variances, reducing the accuracy of the overall measured power values of the transfer matrix. Hence, it becomes relatively difficult to test the device 10 when the test coupler 18 and the fusion splice 20a introduce losses on the same order of magnitude as the optical characteristic being measured.
Optical switches have been used to test long term optical characteristics during environmental testing, for example long term insertion loss of an optical device. FIG. 2 is a diagram illustrating uses of 1.times.4 switches in a conventional arrangement to test long term variations. The test equipment 14, 16, 22, and 26 of FIG. 2 is maintained in a controlled environment, where conditions such as temperature, humidity, etc. are maintained at a stable level to determine long term variations in the device under test 10. The device under test 10, however, may be subject to environmental changes to determine long term reliability.
Switch 22a has fiber optic leads 24a, 24b, 24c, and 24d, and switch 22b has leads 24e, 24f, 24g, and 24h. The fiber optic coupler 10 has optical fibers 12a, 12b, 12c, and 12d coupled to leads 24a, 24b, 24g, and 24g via fusion splices 20a, 20b, 20c, and 20d, respectively. The laser 14 is connected to the switch 22a via the coupler 18, and the second end of the coupler 18 is connected to the lead 24e of switch 22b via a fusion splice 20g. The lead 24c of switch 22a is connected to the lead 24f of switch 22b via fusion splices 20e and 20f. The switch 22a is also connected to a reference reflector 26, used for measuring reflected light for calibration purposes.
The arrangement of FIG. 2 tests for back reflectance using the coupler 18 by calibrating the setup using the reflector 26, where laser light passing from the laser 14 to the switch 22a via the coupler 18 is directed to the lead 24d. The reflector 26 reflects the light back to the switch 22a via the lead 24d, and the switch redirects the reflected light back to the coupler 18. The coupler 18, having once reduced the input laser light by -3 dB, adds an additional -3 dB loss as the reflected light is split by the coupler and supplied in part to the laser 14 and the lead 24e for detection by the detector 16 after having passed through the switch 22b. Leads 24c and 24f are used to calibrate the losses due to the coupler 18 and the switches 22a and 22b before passing to the detector 16.
The system of FIG. 2 does not permit the position of the source or detector to be changed. Since laser light is passed through only one of the leads 24a, 24b, 24c, or 24d at a time, the system of FIG. 2 does not provide any arrangement for testing the fiber optic coupler 10 by inputting light into the leads 12c and 12d of the coupler 10. Hence, the arrangement of FIG. 2 is not readily practical for use in a production environment, where a fiber optic coupler needs to be tested reliably and in a time-efficient manner, since the leads of the fiber optic device 10 still need to be disconnected and respliced to fully test the fiber optic device. Hence, variations are still present due to resplicing efforts.